1. Field of the Invention
The present invention relates to an imaging device that converts an image into an electric signal and an imaging apparatus using the imaging device.
2. Description of the Related Art
A common imaging device widely used heretofore includes three kinds of color filters of R, G, and B. Generally, the R, G, B color filters transmit light in an IR wavelength range. Therefore, when the light transmitted through the R, G, B color filters is detected by a photoelectric conversion element to generate R, G, B color image data, color reproducibility thereof is low.
Therefore, in the imaging device using the R, G, B color filters, an IR cut filter is provided in front of the imaging device so that the light transmitted through the R, G, and B color filters does not include the light in the IR wavelength range, thereby improving the color reproducibility. However, the IR cut filter not only cuts the light in the IR wavelength range but also attenuates visible light by about 10% to 20%. Therefore, intensity of light in a visible wavelength range entering into a light receiving pixel decreases, to thereby reduce sensitivity.
As a corrective action against these problems, there is such a method that an imaging device in which half the number of G filters in a Bayer array is replaced by IR filters corrects a signal output from an IR light receiving pixel as a signal amount generated in each light receiving pixel due to the light in the IR wavelength range. This method can improve reduction in sensitivity, while maintaining the color reproducibility. However, because the number of G pixels decreases, resolution is reduced. Further, because the signal is offset as the signal amount generated in each light receiving pixel due to the light in the IR wavelength range at the time of correction, the signal-to-noise (S/N) ratio decreases.
As an application of the imaging device, an imaging device as shown in FIG. 28 has been proposed. This is an imaging device in which a portion of R filters in the Bayer array is selectively replaced by the IR filters, which performs the correction (see Japanese Patent Application Laid-open No. 2006-237737). In this case, although the number of R pixels decreases, reduction of the resolution is relatively small, because of a principle that even if the number of R pixels is increased, the resolution can be hardly improved. However, reduction of the S/N ratio cannot be avoided due to the same reason at the time of correction.
As another corrective action, an imaging device having a configuration including R, G, B color filters, a photoelectric conversion layer, and photoelectric conversion elements sequentially in an optical path guided from an optical system, and an image processor using the imaging device have been designed. In this image processor, at least the R filter also transmits the light in the IR wavelength range, the photoelectric conversion layer absorbs the light in the IR wavelength range to generate electric charge corresponding thereto and transmits light in other ranges, and some of the photoelectric conversion elements correspond to the R filter. This is a method of using R, G, B color image signals and a signal of the light in the IR wavelength range output from the imaging device to perform image processing, thereby improving the color reproducibility of the color image signal (see Japanese Patent Application Laid-open No. 2008-085807).
In this method, reduction of sensitivity and resolution is prevented, while maintaining the color reproducibility. However, because arithmetic (subtraction) processing for the R, G, B color image signals and the signal of the light in the IR wavelength range is performed by the image processing, the S/N ratio decreases. Further, because the photoelectric conversion layer needs to satisfy a composition with the corresponding characteristic, and the imaging device including the photoelectric conversion layer has a complicated structure, it is hard to say that this method is optimal.
Further, in any method described above, an amount of light is absorbed by the R, G, and B color filters. For example, the R filter absorbs light in a blue wavelength range and light in a green wavelength range. Currently, reduction in the sensitivity due to absorption of light cannot be avoided.
Thus, there has been no imaging device that has all of satisfactory color reproducibility, sensitivity, and resolution.
Recently, demands for a wide-angle small imaging apparatus have been increasing for an application such as a back monitor of a vehicle. However, it is difficult to design a small optical system with a small aberration and distortion, and performance needs to be improved in combination with image processing. For example, in Japanese Patent Application Laid-open No. 2006-345054 is described a method of correcting a magnification chromatic aberration generated in an optical system, in an imaging apparatus using an optical system having a distortion, by performing coordinate transformation for each of R (red), G (green), and B (blue) signals obtained by an imaging device such as a charge coupled device (CCD) and a complimentary metal oxide semiconductor (CMOS) to generate different distortion.
Generally, a color filter such as of a Bayer array is provided in the imaging device. In the conventional art, after a defective pixel due to the color filter array such as the Bayer array is interpolated, the magnification chromatic aberration is corrected with respect to a signal obtained by the imaging device.
However, to correspond to an optical system having a large magnification chromatic aberration, a quite large memory capacity is required for the magnification chromatic aberration (for coordinate transformation), and the device becomes very expensive. Further, because the magnification chromatic aberration is different for each color component, a memory capable of addressing independently for each color component is required for correction of the magnification chromatic aberration, and it is necessary to use an expensive 3-port random access memory (RAM) (for example, a static RAM (SRAM)) having a 3-chip configuration or to drive a RAM in a time-sharing manner, which makes the device even more expensive.